1. Field of the Invention
The present invention relates to a semiconductor producing method of forming a highly uniform crystalline silicon film in processes relating to manufacture of insulated-gate semiconductor devices such as thin-film transistors (TFTs) which are formed by using a non-single-crystal, crystalline silicon film provided on a glass substrate, and other semiconductor devices. In particular, the invention is effective in forming a semiconductor device on a glass substrate.
2. Description of the Related Art
In recent years, insulated-gate field-effect transistors having a thin-film active layer (or active region) on an insulative substrate, i.e., thin-film transistors (TFTs), have been studied eagerly.
The TFTs are classified into an amorphous silicon TFT, a crystalline silicon TFT, etc. by the semiconductor material used and its crystal state. The xe2x80x9ccrystalline siliconxe2x80x9d does not always mean single crystal silicon but may mean non-single crystal silicon in some cases. TFTs using the latter are generally called non-single-crystal silicon TFTs.
In general, amorphous semiconductors have a small electric field mobility, which therefore cannot be used for TFTs that are required to operate at high speed. Further, amorphous silicon can provide only a very small P-type electric field mobility, it does not allow formation of a P-channel TFT (i.e., PMOS TFT), so that a complementary MOS (CMOS) circuit cannot be formed by combining P-channel TFTs and N-channel TFTs (NMOS TFTs).
In contrast, since crystalline semiconductors have larger electric field mobilities than amorphous semiconductors, they allows high-speed operation of TFTs. Further, allowing formation of not only an NMOS TFT but also a PMOS TFT, crystalline silicon enables formation of a CMOS circuit.
A non-single-crystal silicon film is obtained by forming an amorphous silicon film by vapor-phase growth and then thermally annealing it for a long time at a proper temperature (usually higher than 600xc2x0 C.) or irradiating it with strong light such as laser light (optical annealing).
However, where a glass substrate, which is inexpensive and highly workable, is used as an insulative substrate, it is very difficult to form, only by thermal annealing, a crystalline silicon film having a sufficiently large electric field mobility (to allow formation of a CMOS circuit). This is because a glass substrate generally has a low strain temperature (about 600xc2x0 C.) and therefore it is distorted when its temperature is increased to a value that is necessary to form a crystalline silicon film having a sufficiently high mobility.
On the other hand, where optical annealing is used to crystalline a silicon film formed on a glass substrate, high energy can be applied to only the silicon film without much increasing the temperature of the substrate. The optical annealing is thus very effective for crystallization of a silicon film formed on a glass substrate.
At present, large output pulsed lasers such as excimer lasers are considered most suitable for a light source for optical annealing. Since those lasers have much larger maximum energies than CW lasers such as an argon ion laser, they allow use of a beam spot as large as several square centimeters, thus contributing to increase of the productivity.
However, to process a large-area substrate with an ordinary square or rectangular beam, it is necessary to move the beam in the two orthogonal directions. This is an item to be improved to increase the productivity.
This item can be greatly improved by deforming a beam into a linear shape that is longer than the width of a substrate to be processed and scanning the substrate with the beam relatively. (The scanning is performed by moving a linear laser beam in small steps with overlaps.) Details are described in Japanese Unexamined Patent Publication No. 5-112355.
A crystalline silicon film having a higher degree of crystallinity can be obtained by performing thermal annealing before the optical annealing. As for the method of thermal annealing, as disclosed in Japanese Unexamined Patent Publication No. 6-244104, a crystalline silicon film can be obtained at a lower temperature and in a shorter time than the case of using ordinary thermal annealing by utilizing the fact that such elements as nickel, iron, cobalt, platinum, palladium (hereinafter referred to as xe2x80x9ccrystallization catalyst elementsxe2x80x9d or simply xe2x80x9ccatalyst elementsxe2x80x9d) have an effect of accelerating crystallization of amorphous silicon.
TFTs were formed in matrix form by using a crystalline silicon film formed by a conventional method in which an amorphous silicon film was formed on a glass substrate, annealed, and then subjected to laser annealing with a linear laser beam, and a distribution of their threshold voltages in the substrate surface was examined, which is shown in FIG. 2. It is seen from FIG. 2 that the distribution assumes a U-shape.
FIG. 4 shows an arrangement of TFTs on a glass substrate. In FIG. 4, TFTs are arranged in a matrix of 400xc3x97300 in an area of 40 mmxc3x9750 mm of a 100 mmxc3x97100 mm Corning 7059 substrate. In the data of FIG. 2, the horizontal axis shows positions of 400 TFTs on a horizontal full row (enclosed by a broken line in FIG. 4) of the substrate at the center in the vertical direction.
If TFTs of a pixel matrix that constitutes a pixel area of a liquid crystal display has the distribution of threshold voltages as shown in FIG. 2, there may occur display unevenness or an image defect.
An investigation into the cause of the above U-shaped distribution of threshold voltages in the substrate surface has revealed that it is very similar to a warp in a substrate immediately before application of laser light.
It has also been found that a glass substrate does not have a warp immediately after formation of an amorphous silicon film thereon and a warp occurs due to the fact that the silicon film contracts more than the glass substrate when the substrate is cooled after a heat treatment for crystallizing the amorphous silicon film by solid-phase growth. The warp occurs so as to be concave toward the film forming surface of a substrate.
FIG. 3 shows how laser annealing is performed with linear laser light 2 on a silicon substrate formed on a warped glass substrate 1. In FIG. 3, if the warped substrate 1 is subjected to laser annealing, the substrate surface deviates from focuses 3 of laser light differently at respective positions. It is considered that these deviations cause the silicon film to have different degrees of crystallinity in the substrate surface, so that threshold voltages exhibit a particular distribution in the substrate surface.
In the substrate with which the data of FIG. 2 was obtained, the overall depth of the U shape of the warped substrate was about 50 xcexcm immediately before the application of laser light.
The degree of warp depends on the temperature and time of the heat treatment, the substrate material, and other factors. In the case of a 100 mmxc3x97100 mm substrate, the depth of the U shape generally fell within the range of 20 to 200 xcexcm.
A concave warp occurred not only after the thermal crystallization of an amorphous silicon film formed on a glass substrate but also after slow cooling that was performed after an amorphous silicon film was subjected to laser annealing while being heated.
An object of the present invention is to provide a method for forming a crystalline silicon film on a glass substrate which film has a uniform distribution of crystallinity in the substrate surface.
Another object of the invention is to provide a method for forming crystalline silicon thin film transistors (TFTs) on a glass substrate which TFTs have a uniform distribution of threshold voltages in the substrate surface.
Another object of the invention is to provide a producing method which provides a uniform distribution of crystallinity in the substrate surface in a process of forming a crystalline silicon film on a glass substrate which process includes a thermal annealing step and a subsequent laser annealing step, and which method forms crystalline silicon TFTs having a uniform distribution of threshold voltages in the substrate surface by using the silicon film thus obtained.
Another object of the invention is to obtain a flat substrate after slow cooling that is performed after an amorphous or crystalline silicon film is subjected to laser annealing while being heated.
To attain the above objects, according to the invention, there is provided a producing method of a semiconductor device, wherein a linear laser beam that is applied to an uneven irradiation surface on which a semiconductor film is formed has a focus line that extends in a longitudinal direction of the linear laser beam and approximately coincides with a sectional shape of the irradiation surface.
According to the invention, there is provided a producing method of a semiconductor device, wherein in irradiating and scanning a semiconductor film formed on an uneven surface with a linear laser beam, the linear laser beam has a focus line that extends in a longitudinal direction thereof and approximately coincides with a sectional shape of the irradiation surface.
According to the invention, there is provided a producing method of a semiconductor device, comprising, in irradiating and scanning an amorphous silicon film formed on a glass substrate with a linear laser beam, the steps of: placing the glass substrate so as to have a convex surface; irradiating and scanning, in a heated state, the amorphous silicon film with the linear laser beam having an inverted-U-shaped focus line that approximately coincides with the convex surface; and performing cooling.
According to the invention, there is provided a producing method of a semiconductor device, comprising, in converting an amorphous silicon film formed on a glass substrate into a crystalline silicon film by heating and irradiating and scanning the crystalline silicon film with a linear laser beam, the steps of: placing the glass substrate so as to have a convex surface; irradiating and scanning, in a heated state, the crystalline silicon film with the linear laser beam having an inverted-U-shaped focus line that approximately coincides with the convex surface; and performing cooling.
In the above producing methods, as the amorphous or crystalline silicon film is irradiated and scanned with the linear laser beam having the inverted-U-shaped focus line, the glass substrate or the focus line of the linear laser beam may be moved in the height direction of the glass substrate in accordance with a variation of the height of the convex surface in the scanning direction.
According to the invention, there is provided a producing method of a semiconductor device, comprising, in irradiating and scanning an amorphous silicon film formed on a glass substrate with a linear laser beam, the steps of: placing the glass substrate so that it assumes an inverted-U-shaped convex surface; irradiating and scanning, in a heated state, the amorphous silicon film with the linear laser beam having an inverted-U-shaped focus line that approximately coincides with the inverted-U-shaped convex surface; and performing cooling. According to still another aspect of the invention, there is provided a producing method of a semiconductor device, comprising, in converting an amorphous silicon film formed on a glass substrate into a crystalline silicon film by heating and irradiating and scanning the crystalline silicon film with a linear laser beam, the steps of: placing the glass substrate so that it assumes an inverted-U-shaped convex surface; irradiating and scanning, in a heated state, the crystalline silicon film with the linear laser beam having an inverted-U-shaped focus line that approximately coincides with the inverted-U-shaped convex surface; and performing cooling.
In the above producing methods, the glass substrate may placed on a stage having a convex surface or an inverted-U-shaped convex surface while end portions of the glass substrate are pressed against the stage.
It is preferred that the heated state is such that the temperature of the glass substrate be kept in a range from a temperature higher than the room temperature to 70% of the strain absolute temperature of the glass substrate.
The heating of the glass substrate may be performed by heating a helium gas with a heater that is provided under the glass substrate and circulating the heated helium gas under the glass substrate.
In each of the above producing methods, it is preferred that the energy density profile of the linear laser beam in its width direction satisfy inequalities 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 where L1 is a beam width at an energy density that is 95% of a maximum energy density and L1+L2+L3 is a beam width at an energy density that is 70% of the maximum energy density, L1 and L3 corresponding to both side portions of the energy density profile. In particular, it is preferred that the linear laser beam have a depth of focus that is about xc2x1400 xcexcm.
As described above, according to the invention, a flat glass substrate on which an amorphous silicon film is formed or a glass substrate that is warped toward the film coating side after thermal crystallization of the amorphous silicon film is placed on the stage having a convex surface or an inverted-U-shaped convex surface so as to conform to such a surface. While this state is maintained and the glass substrate is kept at a particular temperature within a range from a temperature higher than the room temperature to 70% of the strain temperature (absolute temperature) of the glass substrate, a linear laser beam having a focus line that coincides with the curved surface of the glass substrate is applied to the amorphous or crystalline silicon film formed on the glass substrate. Thereafter, cooling is performed.
In the cooling that is performed after the laser light irradiation, the silicon film contracts more than the glass substrate. As a result, the glass substrate is changed from the curved state to a flat state.
In the above manner, a crystalline silicon film having a uniform distribution of crystallinity can be formed and the glass substrate can be made flat. A crystalline silicon film having a uniform mobility distribution in the substrate surface can be obtained. Further, TFTs having uniform characteristics can be obtained, and a liquid crystal electro-optical device using those TFTs can be produced. Since the glass substrate is flat, the liquid crystal electro-optical device can be produced easily with high accuracy.
The convex surface of the stage is so designed that a glass substrate bearing a silicon film is rendered flat by the slow cooling that is performed after the laser light irradiation.
To realize the above producing process, in performing laser annealing by using a linear laser beam, the focus line of the irradiation laser beam is made to coincide with a shape of an irradiation surface (i.e., a sectional shape in the beam longitudinal direction at a linear laser beam irradiating position). Thus, uniform laser annealing is performed on a curved irradiation surface.
FIG. 1 shows an example of a linear laser beam 11 having an inverted-U-shaped focus line. If the irradiation surface has an inverted-U shape 12 as obtained when the substrate is curved in one direction, a linear laser beam 11 having an inverted-U-shaped focus line that conforms to the curved surface in its longitudinal direction is applied to the irradiation surface through an optical system 10 so that the focus line is located on the irradiation surface, as shown in FIG. 1.
FIG. 12 shows an example of a laser light irradiation method. As shown in FIG. 12, while a linear laser beam 21 having an inverted-U-shaped focus line 20 is applied so that the focus line 20 is located on a curved surface of a glass substrate 23 that is placed on a stage 22, the stage 22 is moved relative to the laser beam 21 in its width direction (indicated by an arrow). Thus, laser annealing that is uniform in the substrate surface can be performed on the curved irradiation surface.
On the other hand, if an irradiation surface has not a simple inverted-U shape but a convex shape in which the center of the surface is high and the periphery is low, the irradiation surface has a height difference not only in the longitudinal direction of a linear laser beam but also in its width direction (i.e., movement direction).
FIG. 13 shows another example of a laser light irradiation method. In this case, as a linear laser beam 31 having an inverted-U-shaped focus line is applied, a stage 33 on which a glass substrate 32 is placed is moved in the height direction as well as in the horizontal direction (indicated by arrows) so that the irradiation surface always coincides with the focus position (indicated by a broken line) of the laser beam 31.
The position of the focus of the laser beam 31 may be controlled by adjusting the lens position while the height of the stage 33 on which the substrate 32 is placed is fixed.
The above control operations may be performed based on such data as a known thickness of the substrate and the shape of a convex surface. Alternatively, the height of the substrate 32 or the focus line may be changed automatically based on the height variation of the irradiation surface which is measured with a laser displacement meter or the like.
To use a linear laser beam having an inverted-U-shaped focus line in its longitudinal direction in performing laser annealing on a glass substrate that is forced to assume a convex surface, a cylindrical lens through which the linear laser beam passes immediately before striking the irradiation surface should have different focal lengths in the longitudinal direction; for instance, it should have an inverted-U-shaped cross-section.
FIG. 9 shows a cylindrical lens 41 having different focal lengths in the longitudinal direction, which is composed of a plurality of cylindrical lenses 41a to 41e having different focal lengths.
FIG. 10 shows another cylindrical lens 42 having different focal lengths in the longitudinal direction, which is constructed such that the plurality of cylindrical lenses 41a to 41e of FIG. 9 are smoothly connected to each other. The cylindrical lens 42 can provide a finer focus line. The cylindrical lens 42 can also provide a variety of focus line shapes other than the inverted-U shape to accommodate various surface shapes of irradiation objects.
In addition to the above producing methods, it is effective to make a laser beam have the following energy density profile.
FIGS. 15A and 15B show laser beam energy profiles. In the invention, a laser beam may have, at the focus, not only a conventional, ordinary rectangular energy density profile shown in FIG. 15A in the width direction but also a trapezoidal one shown in FIG. 15B.
In the laser beam energy density profiles of FIGS. 15A and 15B, with an assumption that the maximum energy density of a laser beam is 1, a beam width at an energy density of 0.95 is represented by L1 and a beam width at an energy density 0.75 is represented by L1+L2 and L3 where L2 and L3 correspond to both side portions of the beam width.
According to the above definitions, a laser beam having a rectangular energy density profile satisfies 0.5L1 greater than L2 (or L3). L2 and L3 are not shown in FIG. 15A because they are very small.
Although a rectangular laser beam has a high energy density on the irradiation surface, its depth of focus is narrower than about xc2x1200 xcexcm. Where the irradiation surface has asperities or undulation, a rectangular laser beam more likely cause a non-uniform distribution of crystallinity than a laser bean having the above-mentioned trapezoidal energy density profile.
On the other hand, the energy density profile shown in FIG. 15B satisfies both inequalities 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 in the width direction of a linear laser beam.
A laser beam having the trapezoidal energy density profile can have a depth of focus that is narrower than about xc2x1400 xcexcm. Where the irradiation surface has asperities or undulation, a laser beam having the trapezoidal energy density profile provides a higher degree of uniformity than a laser beam having the rectangular energy density profile. Further, a trapezoidal laser beam can provide an energy density sufficient for crystallization.
Although a trapezoidal or triangular energy density profile where L2 (or L3) greater than L1 can provide a depth of focus wider than xc2x1400 xcexcm, it is associated with a problem of difficulty in focus adjustment. Further, since the energy density is low, it is likely that the crystallization of a silicon film becomes insufficient, in which case a desired mobility may not be obtained.
Thus, in irradiating a convex surface or an inverted-U-shaped convex surface with a linear laser beam having an inverted-U-shaped focus line, a depth of focus wider than in the case of using a laser beam having the conventional rectangular energy density profile can be obtained by employing a laser beam having the trapezoidal energy density profile in the width direction which satisfies the above inequalities.
FIGS. 16A and 16B show an example of a laser light irradiation method with a linear laser beam having the trapezoidal energy density profile.
In FIG. 16A, by giving the trapezoidal energy density profile to a linear laser beam 51 having an inverted-U-shaped focus line, a silicon film formed on a substrate 53 can be crystallized sufficiently and uniformly with only horizontal movement of a stage 52 if the height difference due to asperities, undulation, etc. of the irradiation surface of the substrate 53 falls within the range of xc2x1400 xcexcm. That is, it is not necessary to move the substrate 53 in the height direction, contributing to simplification of the apparatus and reduction of the cost.
If a laser beam having the trapezoidal energy density profile is used in addition to moving the substrate in the height direction, the focusing margin in the height direction can be increased from the conventional case. Naturally the focusing margin with respect to a height difference of the irradiation surface in the longitudinal direction of a linear laser beam can be increased.
Where the irradiation surface is a convex surface that is not a clear inverted-U shape, the shape of the irradiation surface may not be straight in the substrate movement direction. Therefore, there is a possibility that even if a laser beam has an inverted-U-shaped focus line, it comes out of focus with the irradiation surface.
To solve this problem, a laser beam having a trapezoidal energy density profile and a wide depth of focus is used. As a result, as shown in FIG. 16B, with an inverted-U-shaped focus line 62 of a laser beam 61, the margin is increased as much as an increase in the depth of focus 63, so that the uniformity in the longitudinal direction (horizontal direction on the paper surface of FIG. 16B) can be improved.
When a linear laser beam having a trapezoidal energy density profile is moved relative to the substrate in the width direction of the laser beam (i.e., perpendicularly to its longitudinal direction) with overlaps of beams, due to gradients in the energy density profile of the laser beam, an arbitrary point on the irradiation surface is first irradiated with weak laser beams. Thereafter, at that point, the intensity of laser beams gradually increases, and then gradually decreases, thus completing the irradiating operation.
FIG. 17 shows how laser light irradiation is performed with a linear laser beam having a trapezoidal energy density profile. The laser beam energy density gradually increases in the head portion of the laser light irradiation (indicated by A in FIG. 17) and gradually decreases in the end portion (indicated by B in FIG. 17). Thus, when a linear laser beam having the above trapezoidal energy density profile is used, the variation in the density of energy supplied to the irradiation area becomes much gentler than in the case of using a linear laser beam having the conventional rectangular energy density profile.
This type of laser light irradiation can provide effects equivalent to those obtained by the conventional two-step irradiation in which low-energy-density laser light is applied first (preliminary irradiation) and then high-energy-density laser light is applied (main irradiation).
As a result, abrupt phase changes can be prevented from occurring in a laser-light-irradiated amorphous silicon film, whereby surface roughening and accumulation of internal stress can be prevented. Thus, a uniform distribution of crystallinity can be attained.
In the invention, a substrate heating method as shown in FIG. 14 enables efficient heating during the laser light irradiation. That is, in a state that a substrate 71 is fixed to a stage 73 by means of holding members 72, a helium gas 75 is heated with a heater 74 that is installed under the stage 73 and the heated helium gas 75 is circulated under the substrate 71 by a pump 76, whereby the substrate 71 can be kept at a desired temperature. The reason for using the helium gas 75 is its large heat conductivity.
The inventors checked influences on the substrate shape of every producing step for forming TFTs on a substrate, and found that substrate deformation due to a heat treatment for crystallizing a silicon film was most remarkable. No marked deformation was found after that step.
Therefore, if a substrate is very flat after thermal crystallization or laser light irradiation in a heated state, the substrate can be kept in a fairly flat state even after completion of the entire process. For this reason, by performing laser annealing on a silicon film that is formed on a glass substrate according to the method of the invention, a crystalline silicon film that is flat and has a highly uniform distribution of crystallinity in the substrate surface.
By forming a number of TFTs by using the crystalline silicon film, the distribution of threshold voltages of the TFTs can be made very uniform in the substrate surface. This effect becomes more remarkable as the substrate area increases.
Where a glass substrate has a size of about 100 mmxc3x97100 mm and a thickness of about 1 mm, the convex surface of the stage is such that the height difference of the convex surface between a central portion and end portions (lowest portions) in the area covered by glass substrate is 20 to 200 xcexcm, for instance, about 50 xcexcm.
Where a glass substrate has a size of about 500 mmxc3x97500 mm (for instance, 370 mmxc3x97400 mm, 400 mmxc3x97500 mm, or 550 mmxc3x97650 mm) and a thickness of about 0.5 to 0.7 mm, the convex surface of the stage is such that the height difference of the convex surface between a central portion and end portions (lowest portions) in the area covered by glass substrate is about 1 to 2 mm.
If a liquid crystal display is formed by using a glass substrate on which crystalline silicon TFTs for pixels or driving have been formed according to the method of the invention, the cell assembling can be performed easily and positively by virtue of the glass substrate having a very high degree of flatness. In this case, the flattening of a substrate, which is the advantageous effect of the invention, is still effective even if there is no crystallization step with laser light irradiation after the thermal crystallization.
According to the invention, where a glass substrate has a size of 100 mmxc3x97100 mm and a thickness of 1.1 mm, the height difference due to surface roughening and undulation of the glass substrate can be made less than about 10 xcexcm.